1. Field of the Invention
The present invention relates to a subscriber connection circuit.
2. Description of the Prior Art
In known telephone systems, the operating states of the subscriber line loop, which are to be monitored, are determined by a central analysis device which controls a plurality of subscriber connection lines. Depending upon the type of operating state monitoring which is to be effected, this analysis device is connected via a special resistance managing network to the subscriber connection line in question which is to be monitored, and, in addition, the subscriber connection line is fed via special feed resistances which are matched to the monitoring function in question. Although this is a favorable solution from the standpoint of circuitry expense, both as the result of the operation of a multiplexer which serves to assign the analysis device to the individual subscriber connection lines and in the event of transfer from one mode of indication to another, fault pulses can occur which reduce the reliability of the analysis.
Therefore, it has been proposed in the German patent application No. P 30 36 686.0 to provide individual subscriber analysis devices.
In order to limit the extra expense which results from this decentralization, measures are provided which permit one and the same analysis device to be used for more than one type of monitoring function. These measures consist in the provision of a plurality of feed resistors which, depending upon the type of monitoring function to be effected, are connected to be included into the feeding circuit or else to be a part of voltage dividers by way of which the multiply exploited analysis device in question is connected to the subscriber connection line. In this manner, in the proposed subscriber connection circuit, by selecting between low-ohmic feed and high-ohmic feed and a corresponding voltage divider ratio of voltage dividers by which a multiply-exploited analysis device is connected to the subscriber connection line, it is possible to use this analysis device both to monitor the excitation state and the subscriber loop interruptions as a result of selection and clearance.
The above-mentioned German patent application contains a special analysis device for monitoring the loop closure as the result of a call, in particular because of the unreliability of analysis caused by ringing alternating currents which are superposed upon the feed current. This analysis device is connected via two voltage dividers possessing the same divider ratio to the V-wire of the subscriber connection line, one of which voltage dividers contains a capacitor which is arranged in series with its other voltage divider resistors. As soon as the subscriber line loop has opened in the ring phase and therefore only the ringing alternating currents flow on the subscriber line, the inverting and non-inverting inputs of a comparator which forms the analysis device is fed with alternating voltages of the same amplitude which therefore compensate one another so that, in this case, the ringing alternating voltage has no effect upon the analysis device. If, in the event of a loop closure the feed current is superposed upon the ringing alternating current, the potentials at the inputs of the comparator of the analysis device assume different values since the direct current component of the current on the subscriber connection line can have no influence upon the first input because of the aforementioned capacitor.
However, the RC element which precedes the first input of the comparator produces a certain response delay of the analysis device which is undesirable since, in the case of a call, loop closure must be indicated particularly rapidly in order to prevent the sum current consisting of feed current and ringing current which then flows across the microphone of the subscriber station from leading to damage.
If a ground closure of the B-wire occurs as a result of a fault, in this proposed subscriber connection circuit a loop closure can no longer be recognized by the analysis device connected to this line wire which increases the danger that the microphone may be damaged by an excess current during the ringing phase as a result of the increased feed current.